The pattern is preserved in one location inside the inner product
argument, where we instead desire to avoid allocations by collapsing
p_prime into itself. Using `+=` here requires both mutable and immutable
borrows simultaneously, and assigning temporaries to avoid this makes
the implementation less clear.
This changes variable names in the multiopen and commitment opening implementations
and the book's protocol description to keep names and indicies consistent with one
another.
Co-Authored-By: Jack Grigg <jack@electriccoin.co>